1. Field of the Invention
The present invention relates to a fixing device for fixing an electrophotographic monochrome toner image on an image support member such as paper and also to an electrophotographic apparatus using such a fixing device.
2. Description of the related Art
Various electrophotographic processes and methods have been proposed in, for example, Japanese Patent Examined Publication Nos. 42-23910 and 43-24748 and U.S. Patent Specification No. 2,297,691. An electrophotographic process which is used most commonly has the steps of forming an electrostatic latent image on the surface of a photosensitive member made of a photoconductive material, developing the latent image into a visible image by means of a toner and transferring the developed image onto an image supporting member such as paper.
Many dry developing methods have been known for developing an electrostatic latent image by using toners, such as powder cloud method disclosed in U.S. Pat. No. 2,221,776, cascade method disclosed in U.S. Pat. No. 2,618,552, magnetic brush method disclosed in U.S. Pat. No. 2,874,063, fur brush method disclosed in U.S. Pat. No. 2,901,974, contact developing method disclosed in U.S. Pat. No. 2,811,465, pressurized developing method disclosed in U.S. Pat. No. 3,152,012, and magnetic toner developing method disclosed in U.S. Pat. No. 3,909,258. Among these known developing methods, a cascade method and a magnetic brush method are used most commonly.
Methods are also known for fixing a toner image to an image supporting member such as paper after the transfer of the toner image thereto. For example methods called a solvent fixing method, a pressure fixing method and a heat fixing method have been known. The solvent fixing method, however, has problems such as generation of offensive odor and undesirable effect on human health, due to scattering of vapor of the solvent. The pressure fixing method also is disadvantageous because of unsatisfactory fixing characteristics.
In view of the above-described shortcomings of the prior art, as well as safety and economy, a fixing device of the type called a thermal fixing device has been used, wherein the toner forming a toner image is thermally fused so as to adhere to an image supporting member such as paper. More particularly, as shown in FIG. 4, a fixing roller 1 is composed of a cylindrical core 2 made of a metal such as an aluminum alloy or ceramics and a heater 5 disposed in the cylindrical core 2, the cylindrical core 2 having a surface layer 4 made of a heat-resistant resin such as polytetra fluoroethylene or a heat-resistant elastomer material such as silicone rubber or a fluoric rubber. The fixing device further has a pressing roller 6 composed of a cylindrical core 2' made of, for example, aluminum and having a surface layer 4' made of an elastic material such as silicone rubber or a fluoric rubber or a heat-resistant resin such as fluororesin.
The fixing device further has a plurality of separation claws 18 which prevent coiling of the paper around the fixing roller or the pressing roller. For example, four to five separation claws, each having a width of several millimeters, are used. In most cases, the separation claws are provided only on the fixing roller. In case of a high-speed printing machine or a machine having double-side printing function, however, the separation claws are arranged also on the pressing roller 6.
In operation, the transfer paper 15 (image supporting member) which carries a toner image transferred thereto, passes through the nip between the fixing roller 1 and the pressing roller 6, so that the toner image is heated and pressured to be fixed on the transfer paper 15. This fixing method is generally referred to as heat roller fixing method. Although other types of thermal fixing method such as flash fixing method and oven fixing method are also known, the heat roller fixing method is used most commonly because it excels in fixing performance, fixing speed, safety and economy.
In general, an image exhibits gloss after it is fixed by a conventional fixing device having a fixing roller with a heat-resistant resin surface layer 4. In particular, monochromatic image after such fixing is not easily visible because of reflection of light. This heat roller fixing method employing this type of fixing roller could not provide fixed image of high quality. In general, the fixing roller having a surface layer made of a fluororesin is produced by heating the fluororesin to melt it and applying the melt on the surface of the cylindrical core 2, followed by polishing of the coating surface layer by buffing. Consequently, the surface of the layer 4 is highly smooth and has only a small degree of elasticity. Consequently, the toner particles forming the toner image on the image supporting member 15 are squeezed into flat state, so that the fixed image becomes glossy.
The above-mentioned problem due to gloss of the fixed image is not so serious when the surface layer 4 of the fixing roller is made of an elastic material such as a silicone rubber. Namely, in such a case, partly because fine irregularities are formed on the surface of the layer 4 and partly because the surface layer has appreciable elasticity, the toner particles are not squeezed, so that the image after the fixing does not have any gloss.
The elastic material such as silicone rubber, however, generally exhibits small resistance to wear, so that the surface layer 4 of the fixing roller tends to be worn down rapidly due to frictional contact with the image supporting member 15, separation claws 18 and a temperature sensor 19, so that the fixing performance is impaired in a short period of time, resulting in a short life of the fixing roller.
The elastic surface layer is generally not resistant to mechanical compression so that the surface layer is made to have a large thickness in order to attain a sufficient resistance to compression. Such a large thickness of the surface layer reduces transfer of heat from the heater 5 to the surface of the surface layer, i.e., the surface of the fixing roller 1. Consequently, the temperature of the surface of the fixing roller 1 is undesirably lowered to a level below that required for the fixing, particularly when a number of consecutive image supporting members are made to pass through the fixing device, with the result that the fixing performance is seriously deteriorated. Consequently, the elastic member tends to swell and deform due to presence of a parting agent such as silicone oil which is applied to prevent toner offset, causing wrinkling of the image supporting member 15 such as a paper. The use of such a parting agent is necessary because omission of such agent undesirably not only allows occurrence of toner offset but tends to cause part of the toner on the image supporting member 15 to adhere to the surface layer 4 of the fixing roller 1 so as to impede separation of the image supporting member from the fixing roller 1. With this type of fixing roller, therefore, it is difficult to obtain a maintenance-free type fixing device.
Japanese Patent Unexamined Publication No. 61-32081 discloses a method which employs a specific fixing roller constructed to suppress generation of gloss of fixing images. This fixing roller has a cylindrical core 2 clad with a fluorine-type thermally contracted tube through the intermediary of an adhesive layer. The surface of the tube has been coarsened to a surface roughness of 2 to 3 .mu.m Rz in terms of ten-point average measurement. Fixing rollers, however, are required to have a service life of a period which is 2 to 3 times as long as that of developing agent. Since the coarsened surface of the fixing roller of this type are rapidly worn, this type of roller cannot be satisfactorily used as a fixing roller which is required to provide a number of copies, e.g., several thousands and, in some cases, more than one million. A high-speed fixing essentially requires that the speed of rotation of the fixing roller be increased. Unfortunately, however, the coarsened surface promotes toner offset particularly when the fixing roller is rotated at high speed.
In the field of electrophotography, there have been increasing demands for higher operational speed and improved image quality, as well as for reduction in the power consumption and production cost. In particular, extension of the life of fixing roller and elimination of necessity for maintenance work for fixing roller are matters of great concern.